From DE-A 37 25 290 and from EP-A-0 301 536 it is known that ternary alloys with palladium as the primary metal, another metal from Group VIII of the periodic system, especially nickel, and copper are suitable as a contact catalyst for the oxidation of hydrogen in an atmosphere containing hydrogen and oxygen. A ternary alloy made up of at least 89% by weight of Pd, a maximum of 10% by weight of Ni, and a maximum of 1% by weight of Cu, or in particular 95% by weight of Pd, 4% by weight of Ni, and 1% by weight of Cu, is disclosed in these references as a preferred composition. Such catalysts, in the form of carrier bodies coated with the catalytic alloy, are used in nuclear power plants, for example, where in certain accident situations large amounts of hydrogen are liberated that can be removed with the aid of such catalysts in order to prevent the generation of explosive gas mixtures. However, the problem of removing hydrogen from a gas mixture containing hydrogen and oxygen in order to prevent the danger of an explosion arises in other situations as well, for example in connection with garbage incineration.
From DE-A-36 38 520 a procedure is known for producing a so-called skeleton catalyst or Raney catalyst, in which a mixture of materials in powder form is applied to a carrier element by thermal spraying, with this mixture containing a catalytically effective component and a component that is soluble in an alkaline solution or acid. The soluble component is subsequently dissolved out of the previously applied layer by means of an acid or alkaline solution. The coating can be applied in stages, with a different powder composition each time, so that the innermost layer contains no or only a small portion of the soluble component, whereas in the outermost layer the portion of soluble component is considerably greater than that of the catalytically effective component. In this prior art, the thermal spraying technique is supposed to improve in a simple way the adhesion of the catalytic layer to the carrier element. The dissolving out of the soluble component in the thermally sprayed-on layer makes this technique very troublesome and, depending on the material of the catalytically active component, it can lead to a situation where a chemical reaction takes place with the latter that can impair the catalytic effect.
From EP-A 0 503 470 a procedure is known for producing a metallic catalyst layer on a carrier material, in which Pd, Pt, or a PdNi alloy is applied to a carrier material by means of plasma spraying or flame spraying for the purpose of creating a porous catalyst layer. In this process the carrier material can be preheated before the plasma or flame spraying, and after being coated it can be heat-treated in an atmosphere of hydrogen and an inert gas.
From GB-A-832 031 it is disclosed how to use a two-stage flame spraying procedure in order to produce a catalyst of the platinum group. In this process, first of all aluminum oxide or zirconium oxide is applied by flame spraying to a carrier made of a material with a high melting point, such as silicon carbide, in order to create a rough surface with good adhesive properties. Then likewise by flame spraying the catalyst material, in the form of platinum or a platinum alloy, is applied.
From EP-A-0 172 280 yet another procedure is known for the production of a catalyst with a large surface area, in which the catalyst material proper together with another material is applied to a carrier by means of co-sputtering. The carrier is a substrate consisting of particles, especially those of high-melting oxides, nitrides, or carbides. A composite thin film is sputtered on this substrate, with this film consisting of one or more catalytically active metals such as Pt, Pd, Ag, Au, Re, Rh, Ru, and Ir, and a simultaneously sputtered-on carrier material, namely an oxide, nitride, or carbide.
It is generally known that one prerequisite for high activity in a contact catalyst is a large surface area that can be easily reached; namely, what matters is a fine dispersion and/or an extensive porosity on the part of the catalytically effective material, as is found, for instance, in familiar spongy platinum. If on the other hand the catalytically effective material is disposed in the form of a coating on a carrier body, a good adhesion of the coating to the carrier body is of great importance. Small spalled flakes of a catalyst material used, for example, for the oxidation of hydrogen could reach a temperature leading to the ignition of the surrounding gas mixture because of the exothermic nature of the catalytic reaction. But there are enormous practical difficulties involved in attempting to produce a good adhesion at the same time as a large surface area.
The catalytic effect of a catalyst is greater the higher its temperature is, although of course this must remain below the ignition temperature of a surrounding gas mixture. In the oxidation between hydrogen and oxygen, the catalyst heats up until it reaches an equilibrium state where the heat generated is equal to the heat dissipated by radiation and convection. The time that elapses from the commencement of the catalytic effect to the reaching of this equilibrium state depends, among other things, on the total catalytically active surface area, the superficial extent of the carrier body, which as a rule is metallic, and the total mass or thermal capacity. The larger the superficial extent of the carrier body and the larger its mass, the longer it takes to reach the equilibrium state and the desired working temperature of the catalyst. Therefore, in order to achieve a fast response rate on the part of the catalyst, which is essential especially when it is used, as mentioned above, for the purpose of defusing accident or malfunction situations, the total catalytically effective surface area should be as large as possible in comparison with the superficial extent and mass of the carrier body.
The problem solved by the invention is to develop a procedure that makes it possible to create in a simple way a catalyst layer consisting of a metal alloy that adheres tightly to a carrier body and that has a very large active surface area.